Circuit Description - Ten-Tec SSB-CW-AM Instruction Manual

Model 1254 Receiver

Circuit Description

Before we begin a stage by stage description of the circuitry in
this receiver, let's take a broader view of how shortwave
receivers are designed in general. This should help show how
signals actually make their way from the antenna to the speaker.
Depending on their intended function, radio receivers can be
designed using several different electronic architectures. Very
simple receivers can be built from kits or your own parts using
classic autodyne, TRF, regenerative or direct-conversion designs.
Such receiver projects continue to provide the fun of discovering
how radio originated and how simple it can be.
However, the most common approach used in shortwave
receivers is called the superheterodyne method, usually referred
to as "superhet." In this approach, incoming signals from the
antenna are translated in frequency by a special stage of circuitry
called a mixer. A radio frequency (RF) mixer is essentially a high
speed switching circuit that combines the incoming signals with
a local oscillator (LO) which is generated within the receiver.
This mixing process results in a frequency conversion to an
Intermediate Frequency (IF). A superhet receiver typically may
be single, double or triple conversion: the Model 1254 is a
double-conversion receiver with a 45 MHz first IF and a 455 kHz
second IF.
The simple receiver most closely related to the superhet is the
direct conversion design in which a variable LO is mixed with
incoming signals from the antenna. The resulting "IF" is in the
audio spectrum range and is amplified directly for listening.
The local oscillator must be a very clean and stable sinewave
because any undesirable characteristics on this signal would be
transferred to the received signal by the mixing process. Most
modern receivers use a "synthesizer" to generate this signal. The
synthesizer circuit is able to tune the local oscillator in very
accurate steps and also hold the oscillator on frequency with
very little drift.
Mathematically, the mixer multiplies the instantaneous values of
the LO and the input together. The result is a pair of output
bands that are the sum and difference, in frequency, of the input
and the LO. The LO frequency range is chosen so that one of
these bands can be separated out for further amplification and
1254 - Reference - 6